Many electronic devices on the market today often use power converters to convert electric energy from one form to another (e.g., converting between alternating current and direct current), converting a voltage or current of an electrical signal, modifying a frequency of an electrical signal, or some combination of the above. Examples of power converters may include boost converters and buck converters. Such power converters are often used to convert an input voltage for other circuitry, wherein such converted voltage is greater than (e.g., if a boost converter is used) or less than (e.g., if a buck converter is used) the input voltage. A switching direct current-to-direct current (DC-DC) converter is a type of electronic circuit that converts a source of power from one DC voltage level to another DC voltage level. Examples of such switching DC-DC converters include but are not limited to a boost converter, a buck converter, a buck-boost converter, an inverting buck-boost converter, and other types of switching DC-DC converters.
FIG. 1A illustrates an example synchronous switching DC-DC buck converter 100, as is known in the art. Buck converter 100 may include a switch 1, a switch 2, an inductor 104, and a capacitor 106 coupled in the manner shown in FIG. 1A. When switch 1 is closed, an input voltage supply 102 may provide a DC voltage to inductor 104, and when switch 2 is closed, inductor 104 may discharge to a ground voltage. In typical operation, a switch control circuit controls the turn-on times and turn-off times of switches 1 and 2, and a current is maintained in inductor 104 to transfer energy from the input voltage supply 102 to the output voltage 108, such that output voltage 108 is smaller than the input voltage of input voltage supply 102.
FIG. 1B illustrates an example synchronous switching DC-DC boost converter 100B, as is known in the art. Boost converter 100B may include a switch 1B, a switch 2B, an inductor 104B, and a capacitor 106B coupled in the manner shown in FIG. 1B. An input voltage supply 102B may provide a DC input voltage to boost converter 100B, and in typical operation, a switch control circuit may control the turn-on times and turn-off times of switches 1B and 2B in order to maintain a current in inductor 104B to transfer energy from the input voltage 102B to the output voltage 108B, such that output voltage 108B is larger than the input voltage of input voltage supply 102B.
FIG. 2A illustrates an example inductor current waveform 200 for switching DC-DC converter (e.g., converter 100, converter 100B) operating in continuous conduction mode (“CCM”), as is known in the art. As seen in FIG. 2A, the inductor current is periodic with a switching period T. To regulate an output voltage (e.g., 108), a switch control circuit causes a first switch (e.g., switch 1) to close for a time ton1 while causing a second switch (e.g., switch 2) to remain open, after which the switch control circuit causes the second switch to close for a time ton2 while causing the first switch to remain open, such that T=ton1+ton2. An output voltage Vout (e.g., output voltage 108) and an input voltage Vin (e.g., provided by input voltage source 102) may, in a buck converter, satisfy the relationship Vout/Vin=ton1/(ton1+ton2).
FIG. 2B illustrates an example inductor current waveform 250 for switching DC-DC converter (e.g., converter 100, converter 100B) operating in discontinuous conduction mode (“DCM”), as is known in the art. As seen in FIG. 2B, the inductor current is periodic with a switching period T. To regulate an output voltage (e.g., 108), a switch control circuit causes a first switch (e.g., switch 1) to close for a time ton1 while causing a second switch (e.g., switch 2) to remain open, after which the switch control circuit causes the second switch to close for a time ton2 while causing the first switch to remain open, following which the switch control circuit causes both the first switch and the second switch to open for a time ton3 such that T=ton1+ton2+ton3. For each period T, time duration ton2 is controlled such that switch 2 is kept on until the inductor current decreases to zero and turns off when inductor current decreases to zero to prevent further decrease of the inductor current to a negative value. An output voltage Vout (e.g., output voltage 108) and an input voltage Vin (e.g., provided by input voltage source 102) may, in a buck converter, satisfy the relationship Vout/Vin=ton1/(ton1+ton2).
In order to correctly control switching of the switches shown in the converters of FIGS. 1A and 1B (e.g., buck converter 100 and boost converter 100B), and thus provide a desired output voltage (e.g., 108, 108B), the current through the inductor (e.g., 104, 104B) of the converter may need to be measured so that switches of the converter may be precisely controlled by the switch control circuit. In addition, measurement of current through the inductor of a converter may be used for other purposes, including compensation, operating mode detection, or inductor over-current detection. FIG. 3 depicts an example measurement technique, as is known in the art, detecting a zero crossing of an inductor current in a buck converter. Similar and analogous measurement techniques, although not illustrated herein, may also be applied to other types of converters, including a boost converter, buck-boost converter, and inverted buck-boost converter. As shown in FIG. 3, the inputs of a comparator 302 may be coupled to opposite terminals of switch 2, to measure a voltage across switch 2. In accordance with one existing approach to detect a zero crossing of inductor current, comparator 302 may monitor a voltage across switch 2 when switch 2 is closed, wherein such voltage is induced by a current flowing from inductor 104 through switch 2. If the voltage is positive, indicative of a current flowing from inductor 104 to ground via switch 2, comparator 302 may output a signal (e.g., a logical binary “1”) indicating such positive voltage. On the other hand, if the voltage is negative, indicative of a current flowing from ground to inductor 104 via switch 2, comparator 302 may output a signal (e.g., a logical binary “0”) indicating such negative voltage. A disadvantage of such approach is that it may require a high-precision, high-speed comparator 302. Due to what may be a small resistance of switch 2, the induced voltage drop across switch 2 may also be small, which may impose strict sensitivity requirements on comparator 302. Also, a delay of comparator 302 may cause a time difference between the zero crossing of the inductor current and switch 2 being opened by the switch control circuit. To minimize this time difference, a high-speed comparator may be required.
Another approach which may use the topology shown in FIG. 3 includes closing switch 2 for the duration ton2 and monitoring the output of comparator 302 after switch 2 is turned off. Because the resistance of the switch may be relatively high when it is open, the induced voltage drop across switch 2 may be significantly higher than when switch 2 is closed. Thus, the accuracy and speed requirements of comparator 302 for the approach described above may be alleviated. However, a disadvantage of the latter approach is that under certain conditions, opening switch 2 may stop the flow of inductor current, which may induce lower converter efficiency or cause incorrect operation.